Oxirane compounds such as propylene oxide, and higher alkylene oxide homologs are valuable articles of commerce. One of the most attractive processes for synthesis of these oxirane compounds is described by Kollar in U.S. Pat. No. 3,351,635. According to Kollar, the oxirane compound (e.g., propylene oxide) may be prepared by epoxidation of an olefinically unsaturated compound (e.g., propylene) by use of an organic hydroperoxide and a suitable catalyst such as molybdenum. During the epoxidation reaction the organic hydroperoxide is converted almost quantitatively to the corresponding alcohol. That alcohol may be recovered as a coproduct with the oxirane compound.
Kollar teaches that oxirane compounds may be prepared from a wide variety of olefins. Lower olefins having three or four carbon atoms in an aliphatic chain are advantageously epoxided by the process. The class of olefins commonly termed alpha olefins or primary olefins are epoxidized in a particularly efficient manner by the process. It is known to those in the art that primary olefins, e.g., propylene, butene-1, decene-1, hexadecene-1, etc. are much more difficultly epoxidized than other forms of olefins, excluding only ethylene. Other forms of olefins which are much more easily epoxidized are substituted olefins, alkenes with internal unsaturation, cycloalkenes and the like. Kollar teaches that nowithstanding the relative difficulty in epoxidizing primary olefins, epoxidation proceeds more efficiently when molybdenum catalysts are used. Kollar teaches that activity of certain metals, and particularly molybdenum, for epoxidation of the primary olefins is surprisingly high and can lead to high selectivity of propylene to propylene oxide. These high selectivities are obtained at high conversions of hydroperoxide (50% or higher) which conversion levels are important for commercial utilization of the technology. Kollar's epoxidation reaction proceeds under pressure in the liquid state and, accordingly, a liquid solution of the metal catalyst is preferred. Preparation of suitable catalysts is taught in U.S. Pat. Nos. 3,434,975; 3,453,218; and 3,480,563. These catalysts are produced by the reaction of molybdenum metal or molybdenum oxides with an organic hydroperoxide such as tertiary butyl hydroperoxide in the presence of alcohol or with alcohols. Irrespective, however, of the particular molybdenum compound employed as catalyst in these epoxidation reactions, it has been found that the molybdenum forms a high molecular weight, highly complex compound which because of its low volatility, is carried through the process following successive distillation (or other physical separatory procedure) utilized to recover and separate unreacted olefin, the alkylene oxide product and the byproduct alcohol resulting from the reduction of the organic hydroperoxide.
When an olefin is epoxidized with an organic hydroperoxide in the presence of molybdenum containing catalyst according to the Kollar process, a product mixture containing unreacted alkylene oxide, an alcohol corresponding to the organic hydroperoxide and molybdenum catalyst is obtained. Distillation of that product mixture provides substantially pure alkylene oxide and alcohol products. The residue of diltillation (hereafter "bottoms") contains spent molybdenum catalyst, some alcohol, acids as well as high boiling organic residues. Removal and recovery of the molybdenum values from such organic solutions are important from ecological and economical standpoints and have been the subject of a number of previous researchers.
In U.S. Pat. No. 3,763,303, Khuri et al. disclose two embodiments of a process for recovering molybdenum values from spent epoxidation catalysts. The Khuri process first embodiment involves recovery of molybdenum directly from the spent catalyst mixture by a liquid-to-liquid extraction utilizing an aqueous extractant consisting essentially of water which is intermittently admixed with the residue to be treated to effect an extraction and transfer of a portion of the molybdenum constituent from the organic phase to the aqueous phase. According to Khuri et al, untreated spent catalyst solutions containing molybdenum in concentrations of from about 0.1% to about 1.0%, by weight, are highly satisfactory for treatment in the liquid-to-liquid extraction process in which the extractant consists essentially of water to effect molybdenum separation. Molybdenum separated with the aqueous extract is recovered as molybdenum trioxide by evaporation of water followed by calcination of the solid obtained by extract evaporation.
The second embodiment of the Khuri process relates to extracting molybdenum from distillation residues obtained from distillation of spent catalyst solution (bottoms) but the extraction is performed with acids or bases to convert the molybdenum into a recoverable molybdenum compound of the acid or base.
It has also been suggested in Tave U.S. Pat. No. 3,453,068 to recover molybdenum from such organic solutions by heating the solution in a free oxygen-containing gas to from 850.degree. F. to 2000.degree. F. to convert the molybdenum to the oxide which is collected by cooling to a temperature below the sublimation temperature.
Tave U.S. Pat. No. 3,463,604 describes a process in which the molybdenum contained in the organic residual effluent is precipitated by means of an aqueous solution of ammonium phosphate; in this process an aqueous solution containing ammonium phosphate is admixed with the effluent, thereby precipitating an insoluble phosphomolybdate compound. However, this process is incapable of recovering substantially all of the molybdenum contained in the organic residual effluent, thereby rendering the resultant organic solution freed of molybdenum by the process incapable of use as fuel without subjection to further treatment involving use of expensive apparatus.
British Patent Specification No. 1,317,480 also teaches recovery of molybdenum values from spent epoxidation catalysts. As in Khuri, the British recovery process involves extracting the spent catalysts solution with water alone or with aqueous ammonia. The British extraction process results in a transfer of at least 95% of the available molybdenum values to the aqueous extract. Those molybdenum values are recovered from the aqueous phase by precipitation as a phosphomolybdate or by distillative stripping of the volatile organic material and water from the extract.
The spent catalyst solution may also be subjected to exhaustive evaporation or distillation to produce a residue with a higher molybdenum content as taught by Levine et al. in U.S. Pat. No. 3,819,663. The Levine process starts with a spent catalyst solution, such as the aforedescribed bottoms, and subjects that solution to a wiped film evaporation at 375.degree. F. to 450.degree. F. until 60% to 80% by weight of the solution, is evaporated overhead. The residue of that evaporation is taught to be useful in preparation of a catalyst in further epoxidation processes.
Lemke teaches in U.S. Pat. No. 3,887,361 that molybdenum may be precipitated from spent catalyst solutions obtained from the Kollar epoxidation process by adding tertiary-butyl alcohol to the "bottoms" until a level of 5% to 50%, by weight, of tertiary-butyl alcohol is achieved. Then the mixture is heated to 100.degree. C. to 300.degree. C. to effect molybdenum precipitation; according to the patentee, the precipitated molybdenum can then be reused in further epoxidations. However, this process is quite costly in requiring the use of expensive tertiary butyl alcohol and further, due to the varying nature of the spent catalyst solution to be treated, fails to provide consistent results.